feat: encrypt notification tokens

Author: link2xt

src/config.rs

@@ -441,6 +441,12 @@ pub enum Config {
     /// Enable webxdc realtime features.
     #[strum(props(default = "1"))]
     WebxdcRealtimeEnabled,
+
+    /// Last device token stored on the chatmail server.
+    ///
+    /// If it has not changed, we do not store
+    /// the device token again.
+    DeviceToken,
 }
 
 impl Config {

src/context.rs

@@ -1771,6 +1771,7 @@ mod tests {
             "socks5_password",
             "key_id",
             "webxdc_integration",
+            "device_token",
         ];
         let t = TestContext::new().await;
         let info = t.get_info().await.unwrap();

src/imap.rs

@@ -41,6 +41,7 @@ use crate::mimeparser;
 use crate::net::proxy::ProxyConfig;
 use crate::net::session::SessionStream;
 use crate::oauth2::get_oauth2_access_token;
+use crate::push::encrypt_device_token;
 use crate::receive_imf::{
     from_field_to_contact_id, get_prefetch_parent_message, receive_imf_inner, ReceivedMsg,
 };
@@ -1559,17 +1560,53 @@ impl Session {
             return Ok(());
         };
 
-        if self.can_metadata() && self.can_push() {
+        let device_token_changed = context
+            .get_config(Config::DeviceToken)
+            .await?
+            .map_or(true, |config_token| device_token != config_token);
+
+        if device_token_changed && self.can_metadata() && self.can_push() {
             let folder = context
                 .get_config(Config::ConfiguredInboxFolder)
                 .await?
                 .context("INBOX is not configured")?;
 
-            self.run_command_and_check_ok(format!(
-                "SETMETADATA \"{folder}\" (/private/devicetoken \"{device_token}\")"
-            ))
-            .await
-            .context("SETMETADATA command failed")?;
+            let encrypted_device_token =
+                encrypt_device_token(&device_token).context("Failed to encrypt device token")?;
+
+            // We expect that the server supporting `XDELTAPUSH` capability
+            // has non-synchronizing literals support as well:
+            // <https://www.rfc-editor.org/rfc/rfc7888>.
+            let encrypted_device_token_len = encrypted_device_token.len();
+
+            if encrypted_device_token_len <= 4096 {
+                self.run_command_and_check_ok(&format_setmetadata(
+                    &folder,
+                    &encrypted_device_token,
+                ))
+                .await
+                .context("SETMETADATA command failed")?;
+
+                // Store device token saved on the server
+                // to prevent storing duplicate tokens.
+                // The server cannot deduplicate on its own
+                // because encryption gives a different
+                // result each time.
+                context
+                    .set_config_internal(Config::DeviceToken, Some(&device_token))
+                    .await?;
+            } else {
+                // If Apple or Google (FCM) gives us a very large token,
+                // do not even try to give it to IMAP servers.
+                //
+                // Limit of 4096 is arbitrarily selected
+                // to be the same as required by LITERAL- IMAP extension.
+                //
+                // Dovecot supports LITERAL+ and non-synchronizing literals
+                // of any length, but there is no reason for tokens
+                // to be that large even after OpenPGP encryption.
+                warn!(context, "Device token is too long for LITERAL-, ignoring.");
+            }
             context.push_subscribed.store(true, Ordering::Relaxed);
         } else if !context.push_subscriber.heartbeat_subscribed().await {
             let context = context.clone();
@@ -1581,6 +1618,13 @@ impl Session {
     }
 }
 
+fn format_setmetadata(folder: &str, device_token: &str) -> String {
+    let device_token_len = device_token.len();
+    format!(
+        "SETMETADATA \"{folder}\" (/private/devicetoken {{{device_token_len}+}}\r\n{device_token})"
+    )
+}
+
 impl Session {
     /// Returns success if we successfully set the flag or we otherwise
     /// think add_flag should not be retried: Disconnection during setting
@@ -2864,4 +2908,16 @@ mod tests {
             vec![("INBOX".to_string(), vec![1, 2, 3], "2:3".to_string())]
         );
     }
+
+    #[test]
+    fn test_setmetadata_device_token() {
+        assert_eq!(
+            format_setmetadata("INBOX", "foobarbaz"),
+            "SETMETADATA \"INBOX\" (/private/devicetoken {9+}\r\nfoobarbaz)"
+        );
+        assert_eq!(
+            format_setmetadata("INBOX", "foo\r\nbar\r\nbaz\r\n"),
+            "SETMETADATA \"INBOX\" (/private/devicetoken {15+}\r\nfoo\r\nbar\r\nbaz\r\n)"
+        );
+    }
 }

src/push.rs

@@ -1,10 +1,16 @@
 use std::sync::atomic::Ordering;
 use std::sync::Arc;
 
-use anyhow::Result;
+use anyhow::{Context as _, Result};
+use base64::Engine as _;
+use pgp::crypto::aead::AeadAlgorithm;
+use pgp::crypto::sym::SymmetricKeyAlgorithm;
+use pgp::ser::Serialize;
+use rand::thread_rng;
 use tokio::sync::RwLock;
 
 use crate::context::Context;
+use crate::key::DcKey;
 
 /// Manages subscription to Apple Push Notification services.
 ///
@@ -24,20 +30,85 @@ pub struct PushSubscriber {
     inner: Arc<RwLock<PushSubscriberState>>,
 }
 
+/// The key was generated with
+/// `rsop generate-key --profile rfc9580`
+/// and public key was extracted with `rsop extract-cert`.
+const NOTIFIERS_PUBLIC_KEY: &str = "-----BEGIN PGP PUBLIC KEY BLOCK-----
+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+=5jvt
+-----END PGP PUBLIC KEY BLOCK-----";
+
+/// Pads the token with spaces.
+///
+/// This makes it impossible to tell
+/// if the user is an Apple user with shorter tokens
+/// or FCM user with longer tokens by the length of ciphertext.
+fn pad_device_token(s: &str) -> String {
+    // 512 is larger than any token, tokens seen so far have not been larger than 200 bytes.
+    let expected_len: usize = 512;
+    let payload_len = s.len();
+    let padding_len = expected_len.saturating_sub(payload_len);
+    let padding = " ".repeat(padding_len);
+    let res = format!("{s}{padding}");
+    debug_assert_eq!(res.len(), expected_len);
+    res
+}
+
+/// Encrypts device token with OpenPGP.
+///
+/// The result is base64-encoded and not ASCII armored to avoid dealing with newlines.
+pub(crate) fn encrypt_device_token(device_token: &str) -> Result<String> {
+    let public_key = pgp::composed::SignedPublicKey::from_asc(NOTIFIERS_PUBLIC_KEY)?.0;
+    let encryption_subkey = public_key
+        .public_subkeys
+        .first()
+        .context("No encryption subkey found")?;
+    let padded_device_token = pad_device_token(device_token);
+    let literal_message = pgp::composed::Message::new_literal("", &padded_device_token);
+    let mut rng = thread_rng();
+    let chunk_size = 8;
+
+    let encrypted_message = literal_message.encrypt_to_keys_seipdv2(
+        &mut rng,
+        SymmetricKeyAlgorithm::AES128,
+        AeadAlgorithm::Ocb,
+        chunk_size,
+        &[&encryption_subkey],
+    )?;
+    let encoded_message = encrypted_message.to_bytes()?;
+    Ok(format!(
+        "openpgp:{}",
+        base64::engine::general_purpose::STANDARD.encode(encoded_message)
+    ))
+}
+
 impl PushSubscriber {
     /// Creates new push notification subscriber.
     pub(crate) fn new() -> Self {
         Default::default()
     }
 
-    /// Sets device token for Apple Push Notification service.
+    /// Sets device token for Apple Push Notification service
+    /// or Firebase Cloud Messaging.
     pub(crate) async fn set_device_token(&self, token: &str) {
         self.inner.write().await.device_token = Some(token.to_string());
     }
 
     /// Retrieves device token.
     ///
+    /// The token is encrypted with OpenPGP.
+    ///
     /// Token may be not available if application is not running on Apple platform,
+    /// does not have Google Play services,
     /// failed to register for remote notifications or is in the process of registering.
     ///
     /// IMAP loop should periodically check if device token is available
@@ -121,3 +192,37 @@ impl Context {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_set_device_token() {
+        let push_subscriber = PushSubscriber::new();
+        assert_eq!(push_subscriber.device_token().await, None);
+
+        push_subscriber.set_device_token("some-token").await;
+        let device_token = push_subscriber.device_token().await.unwrap();
+        assert_eq!(device_token, "some-token");
+    }
+
+    #[test]
+    fn test_pad_device_token() {
+        let apple_token = "0155b93b7eb867a0d8b7328b978bb15bf22f70867e39e168d03f199af9496894";
+        assert_eq!(pad_device_token(apple_token).trim(), apple_token);
+    }
+
+    #[test]
+    fn test_encrypt_device_token() {
+        let fcm_token = encrypt_device_token("fcm-chat.delta:c67DVcpVQN2rJHiSszKNDW:APA91bErcJV2b8qG0IT4aiuCqw6Al0_SbydSuz3V0CHBR1X7Fp8YzyvlpxNZIOGYVDFKejZGE1YiGSaqxmkr9ds0DuALmZNDwqIhuZWGKKrs3r7DTSkQ9MQ").unwrap();
+        let fcm_beta_token = encrypt_device_token("fcm-chat.delta.beta:chu-GhZCTLyzq1XseJp3na:APA91bFlsfDawdszWTyOLbxBy7KeRCrYM-SBFqutebF5ix0EZKMuCFUT_Y7R7Ex_eTQG_LbOu3Ky_z5UlTMJtI7ufpIp5wEvsFmVzQcOo3YhrUpbiSVGIlk").unwrap();
+        let apple_token = encrypt_device_token(
+            "0155b93b7eb867a0d8b7328b978bb15bf22f70867e39e168d03f199af9496894",
+        )
+        .unwrap();
+
+        assert_eq!(fcm_token.len(), fcm_beta_token.len());
+        assert_eq!(apple_token.len(), fcm_token.len());
+    }
+}